U. WASHINGTON (US) — New technology could untether cardiac patients as mechanical heart pumps join the next generation of wireless technology.
Ventricular assist devices have traditionally been used as a stopgap measure for patients awaiting heart transplants, but more frequently are operating in patients for years—a development that is not without its own problems.
The cumbersome power cord that protrudes through the patient’s belly becomes infected in close to 40 percent of patients. Infection is the leading cause of rehospitalization and can be fatal.
Researchers have tested a wireless power system that is a variation on inductive power, in which a transmitting coil sends out electromagnetic waves at a certain frequency and a receiving coil absorbs the energy and uses it to charge a battery.
Electric toothbrush charging stations and cell phone charging pads use a similar system, except that in both those cases the tool has to actually touch the charger and be held in a fixed position.
Joshua Smith, associate professor of computer science and electrical engineering at the University of Washington, devised an inductive system that adjusts the frequency and other parameters as the distance or orientation between the transmitter and receiver coils changes, allowing for flexible yet efficient wireless power over medium distances.
“Most people’s intuition about wireless power is that as the receiver gets further away, you get less power,” says Smith, “But with this technique there’s a regime where the efficiency actually doesn’t change with distance.”
The technology was presented recently at the American Society for Artificial Internal Organs annual meeting.
In what Smith calls the “magic regime,” power stays constant over distances about the same as the diameter of the coil—meaning a one-foot transmitter coil could deliver consistent power over a distance of a foot, or a four-inch coil could transmit power over a distance of four inches.
That’s not far, but it’s enough to bridge the skin and tissue to reach a medical implant.
Four years ago, Smith’s system attracted the interest of Pramod Bonde, assistant professor of cardiothoracic surgery at the University of Pittsburgh, who had experimented with using traditional induction to transfer power, but was hampered by misalignment, unwanted heat generation, and ranges that were limited to a few millimeters.
“My primary interest is to help heart failure patients recover, and they can only recover if they are not tethered to a battery or external power supply so they can exercise and train their heart to recover,” Bonde says.
Using the wireless system means no power cord poking through the skin, dramatically reducing the risk of infection and improving the patient’s quality of life.
Researchers envision a vest that could hold an external transmitter coil connected to a power cord or battery. A small receiver coil implanted under the patient’s skin would connect to a battery that holds enough power for about two hours, meaning the patient could be completely free for short periods of time to take a bath or go for a swim, which current users of heart pumps cannot do.
Longer term, additional power transmitters placed under a patient’s bed or chair could allow patients to sleep, work or exercise at home unencumbered.
Test results show the system could power a commercial heart pump running underwater using a receiver coil as small as 4.3 cm (1.7 inches) across. The power transmitted reliably with an efficiency of about 80 percent.
The researchers hope to next test the system with a heart pump implanted in an animal.
“The potential for wireless power in medical fields goes far beyond powering artificial hearts,” Bonde says. “It can be leveraged to simplify sensor systems, to power medical implants, and reduce electrical wiring in day-to-day care of the patients.”
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